O-type traveling wave tube amplifier having means for counteracting the modulation of the spent beam in the collector electrode region



March 18, 1969 J J, T N REm ET AL 3,433,992

O-TYPE TRAVELING wAvE TUBE AMPLIFIER HAVING MEANs FOR COUNTERACTING THE MODULATION OF THE SPENT BEAM IN THE COLLECTOR ELECTRODE REGION Filed June 7. 1966 ON 85mm mofiho dd 9 N mohomjou cwwwmEmm ll ll|| 0m mwizzm wwSE 1m 592322 ww 5o on mokuwjoo Qmmmmmmwo kDQZ. um

INVENTORS, JOHN J. TANCREDI ATTORN EYS United States Patent O-TYPE TRAVELING WAVE TUBE AMPLIFIER HAVING MEANS FOR COUNTERACTING THE MODULATION OF THE SPENT BEAM IN THE COLLECTOR ELECTRODE REGION John J. Tancredi, Neptune, and Bruno W. Zotter, Belmar,

N.J., assignors to the United States of America as represented hy the Secretary of the Army Filed June 7, 1966, Ser. No. 556,819 US. Cl. 315-3.6 8 Claims Int. Cl. H013 25/34, 23/02 ABSTRACT OF THE DISCLOSURE An electron beam microwave interaction device which includes a two-section severed slow-wave circuit both in coupling relationship to the electron beam. One severed circuit section is positioned in the beam path and in coupling relationship to the beam for modulating the beam with an input radio frequency signal such that the radio frequency signal is amplified, and the other severed circuit section is downstream from the first severed section and in coupling relationship to the modulated beam for further amplifying and extracting the signal. A third means which may be either a fast or a slow-wave circuit structure is positioned downstream of the signal extracting means and in coupling relationship to the spent modulated beam exiting from the second severed section. An adjustable phase shifter interconnects the output sever power of the first section to the third means for counteracting the modulation of the spent beam whereby the velocity spread of the beam as intercepted by the collector electrode is reduced.

This invention relates to electron beam devices and more particularly to a means for improving the efficiency of such devices.

In a linear electron beam device such as a traveling wave tube or a klystron, for example, it has been conventional to electrically bias the collector electrode at or very near the potential of the microwave structure of the tube. As is well known, the RF properties of the device require that the microwave structure be at a high potential with respect to the cathode. This in turn requires that the collector also be at a high potential with the result that the electrons in the beam have considerable energy when they strike the collector. This energy is largely converted into heat in the collector and causes considerable power loss and low efficiency operation of the tube. In high power tubes, additional cooling means must be provided to cool the collector. This cooling equipment is often expensive, bulky, and requires a considerable amount of power to operate.

Although the above-noted difficulties could be solved by lowering the potential of the collector, a new problem is created. As the collector potential is lowered, a point is reached where the electrons will be reversed before reaching the collector and, as a result, they are usually intercepted by the higher potential RF structure. The efficiency of the tube is therefore lowered. Additionally, the RF structure may become subject to excessive heating.

It is an object of the present invention to provide a simple means for affecting improvement in the efficiency of the traveling wave tubes. 1

It is yet another object of the present invention to increase the overall efiiciency of traveling wave tubes by utilizing the sever power.

It is still another object of the present invention to provide a highly efiicient traveling wave tube operating with 3,433,992 Patented Mar. 18, 1969 a lower depressed collector potential than heretofore utilized.

In accordance with the present invent-ion there is provided an electron beam microwave energy interaction device which includes means for forming and projecting an electron beam along a linear path and an electron collector electrode disposed along the path in spaced relation to the electron beam forming means. Also included is a two section severed slow-wave circuit. One section of the severed circuit comprises a first means positioned in the linear path and in coupling'relationship to the beam for modulating the beam with an input radio frequency signal wave such that the radio frequency signal is amplified. The other section of the severed slow-wave circuit comprises a second means downstream of the first means and in coupling relationship to the modulated beam for further amplifying and extracting the amplified signal wave. Included further is a third means downstream of the signal extracting means in coupling relationship to the modulated beam and responsive to the sever power for counteracting the modulation of the spent beam exiting from the wave signal extracting means whereby the velocity spread of the beam as intercepted by the collector elect-rode is reduced.

The expression downstream as used herein signifies a closer proximity to the collector electrode than to the electron forming means with respect to a given reference point.

For a better understanding of the invention, together with other and further objects thereof, reference is bad to the following description taken in connection with the accompanying drawing in which:

FIGURE 1 schematically illustrates a preferred embodiment of the invention wherein a slow-wave coupling structure is utilized; and FIGURE 2 is a section of FIG- URE 1 schematically illustrating a fast-wave type coupler in place of the slow-wave coupling structure shown in FIGURE 1.

Referring now to the drawing, there is shown at 10 a traveling wave tube of conventional design. The spaced slow-wave input structure 12 and slow-wave output structure 14 form the two sections of a severed slow-wave circuit and are axially aligned so as to interact with the axial electron beam 16 in the conventional manner. The beam 16 is formed and projected in the usual manner by means of the electron emitting circuit shown at 17. As shown, the input slow-wave structure 12 is severed as at 1-8 and the sever power is coupled through an electronic type RF phase shifter 20 to an axially aligned coupler coil 22, also a slow-wave structure in coupling relationship with the beam 16, and which is positioned between slow-wave output structure 14 and collector 24. Although the structures 12, 14 and coupler coil 22 are shown as helices, it is tobe understood of course that it is not limited thereto and any other suitable slow-wave structures well known in the art may be utilized. As shown, the free end of output structure 14 and the free end of coupler coil 22 are terminated by respective matching load resistors 23 and 25. Although not shown, the respective D-C potentials applied to the input and output RF structures comprising the severed slow-wave circuit are assumed to be conventional. Also, the axial magnetic field, although not shown, is assumed to be conventionally applied. The applied collector electrode 24 D-C potential, however, will be at a much lower magnitude than that conventionally required and preferably placed downstream within a region of minimum electron velocity spread.

In discussing the theory of operation of the invention shown in the drawing, it is to be assumed that an RF input is applied to the slow-wave input structure 12. As in the conventional traveling Wave tube, the unidirectional energy of the electron beam 16 is converted to RF energy through a continuous and cumulative interaction between the beam 16 and a component of the electric field of the RF wave propagated along input RF structure 12. The continuous and cumulative interaction is accomplished by slowing the velocity at which the RF wave progresses axially along the tube to approximate the velocity of the beam 16 and providing a component of electric field in the direction of beam travel. The field component then causes the electrons in the beam to form bunches and effect a modulated beam. The bunches form in a favorable phase of the wave and are slowed by the electric field so that a portion of the beam kinetic energy is thus imparted to the RF wave to produce an amplified signal at the output of input structure 12. The modulated electron beam then interacts with the output RF structure 14 in the usual manner to further amplify the signal and, after the amplified signal is extracted, the spent modulated beam then moves axially along the tube through coupler coil 22 towards collector 24. As described above, the power output derived from sever 18 is fed to the input end of coupler coil 22 through phase shifter 20.

Since the coupler coil 22 is shown as a slow-wave structure, the phase shifter 20 is adjusted such that the energy fed into coupler coil 22 counteracts the phase of the modulated electron beam 16 as it enters and passes through coupler coil 22. The modulation of the electron beam 16 is thereby reduced, as well as the velocity spread of the beam, before it is intercepted by collector electrode 24. In conventional TW tube amplifiers, the input RF structure 12 would normally be terminated by an external sever termination such as a load resistor which, of course, represents wasted power. In the present invention, however, this power is not dissipated but is utilized instead to reduce the velocity spread of the spent beam as it leaves output RF structure 14. With the velocity spread thus reduced, the collector potential may be biased to a relatively low value and if the collector 24 is placed at a region of minimum velocity spread, it can be depressed to an even lower potential. The maximum efficiency improvement which can be achieved is equal to the sever power divided by the collector power. Since the sever power does not have to be dissipated, no output termination for the input RF structure 12 is required, nor are additional cooling channels necessary.

In order to use the large bandwidth of traveling wave tubes, the total delay time through the phase shifter 20 should be the same as that through RF output structure 14. This is achieved by using the same type of circuit for the phase shifter 20 as that of the RF output structure Although the coupled coil 22 is shown as a slow-wave structure, it is not to be limited thereto. A fast-Wave type coupler coil such as a Cuccia coupler for example, could also be used. It is well known that an electron beam which has been modulated by a high frequency electromagnetic wave in effect propagates two space charge waves as a result of the modulation. One of the space charge waves travels at a velocity less than the direct current velocity of the electron beam and is generally referred to as the slow-wave. In the description above the coupler coil 22 of FIG. 1 is a slow-wave interacting structure. However, the other space charge wave travels faster than the direct current beam velocity and is referred to as the fast-wave. When the fast space charge wave propagates on the electron beam, the kinetic power of the beam is greater than the direct current power, which means that in order to excite the fast space charge wave, radio frequency power must be delivered to the beam. In the description above, the velocity spread was reduced by reducing the modulation of the electron beam. The same result can be achieved by coupling to the fast-wave by a suitable fast-wave coupling device such as the Cuccia coupler 22 shown in FIG. 2. In this case the phase shifter 20 would be adjusted so that the power fed into the fast-wave coupler 22 is 4 such that a standing wave is excited as the beam passes therethrough and interacts therewith.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron beam microwave interaction device comprising, means for forming and projecting an electron beam along a path,

an electron collector electrode disposed along said path in spaced relation to said electron beam forming means,

a two-section severed slow-wave circuit,

one section of said severed circuit comprising a first means positioned in said path and in coupling relationship to said beam for modulating said beam with an input radio frequency signal such that said radio frequency signal is amplified,

the other section of said severed circuit comprising a second means downstream of said first means and in coupling relationship to said modulated beam for further amplifying and extracting said signal wave,

a third means downstream of said wave signal extracting means in coupling relationship to the spent modulated beam exiting from said second means and means responsive to the sever power output of said first means and in circuit with said third means for counteracting the modulation of said spent beam whereby the velocity spread of said beam as intercepted by said cellector electrode is reduced.

2. The device in accordance with claim 1 wherein the sever power is applied to the input of said third means through an adjustable phase shifter.

3. The device in accordance with claim 1 wherein said collector electrode is within a region of minimum electron velocity spread.

4. The device as claimed in claim 1 wherein said first, second and third means each comprise a wave propagation structure in slow-wave mode interaction with said beam.

5. The device in accordance with claim 1 wherein said first and second means each comprise a wave propagation structure in slow-wave mode interaction with said beam and said third means is a wave propagation circuit in fast mode interaction with said beam.

6. The device in accordance with claim 4 wherein said collector electrode is within a region of minimum electron velocity spread.

7. The device in accordance with claim 2 wherein said first, second and third means each comprise a wave propagation structure in slow-wave mode interaction with said beam.

8. The device in accordance with claim 2 wherein said first and second means each comprise a wave propagation structure in slow-wave mode interaction with said beam and said third means is a wave propagation circuit in fast mode interaction with said beam.

References Cited UNITED STATES PATENTS 2,767,259 10/1956 Peter 315-3.6 X 2,849,642 8/ 1958 Goodall 315-36 3,144,616 8/1964 Jepsen 315-35 X 3,302,053 l/ 1967 Udelson 3155.38

HERMAN KARL SAAL'BACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 315-538; 330-43 

